The invention relates to an electrical apparatus, particularly a relay or a small-size contactor, comprising a hollow, prism-shaped casing adapted to accomodate a particular device and having a rectangular opening in a lower area thereof opposite to connection terminals, opening which is closed by a generally rectangularly-shaped removable bottom which is provided with fastening means adapted to cooperate with holding means on said casing and with removable hooking means which are disposed externally and are adapted to cooperate with a pair of parallel flanges of a fixed, metal shaped rail or section.